Rigid polyurethane foams are widely known and are used in numerous industries. Rigid polyurethane foams are produced by reacting a polyisocyanate with a polyol in the presence of a blowing agent. Chlorofluorocarbons (CFC's) were typically used as blowing agents to produce rigid polyurethane foams which have excellent insulating properties. CFC's are now believed to contribute to the depletion of ozone in the stratosphere. As a result, mandates have been issued which prohibit the use of CFC's.
Hydrogen-containing chlorofluorocarbons (HCFC's), hydrofluorocarbon compounds (HFC's) and mixtures of HCFC's and HFC's are blowing agents considered to be acceptable alternatives to CFC's. HCFC 141b is currently used as an alternative to CFC's. However, due to the fact that the use of HCFC 141b will be phased-out beginning in 2003, effort has been directed to using water as a blowing agent in the production of some rigid polyurethane foams.
There are, however, drawbacks to using water as a blowing agent for producing rigid polyurethane foams. One such drawback is the fact that foams produced using relatively high levels of water as a blowing agent are friable and have relatively poor adhesion to substrates. See U.S. Pat. No. 5,013,766, column 1, lines 11-13.
A process for producing rigid polyurethane foams which are less brittle and which have acceptable adhesion to substrates has been investigated. For example, U.S. Pat. No. 5,013,766 describes a process for producing closed-cell rigid polyurethane foams which are less friable and adhere well to substrates. The process disclosed in this patent focuses on reacting an isocyanate with a polyol mixture in the presence of a catalyst, water, trichlorofluoromethane (Freon 11) and a foam stabilizer. However, from the examples contained in U.S. Pat. No. 5,013,766, one skilled in the art would recognize that foams produced by the process described in this patent would have low compressive strength.
Open-cell rigid polyurethane foams having acceptable compressive strength which are produced using polymer polyols are known. For example, U.S. Pat. No. 6,127,443 discloses a process for producing open-cell rigid energy absorbing polyurethane foams by reacting certain polymer polyols with an isocyanate in the presence of a blowing agent (water) to generate rigid polyurethane foams in which the total polymer solids content of the foam is in excess of about 15 weight percent. Foams produced by the process described in this patent are designed for energy management. Due to their open-cell structure, one skilled in the art would expect that the foams produced by the process described in U.S. Pat. No. 6,127,443 would have poor insulating properties. See W. A. Kaplan et al., Low-Density All Water-Blown Rigid Foam for Pour-in-Place Applications, Polyurethanes Expo '96 Conference Proceedings, pp. 179-89 (1996) wherein it states that, unlike closed-cell water blown polyurethane foams, open-cell polyurethane foams are poor insulators.
There therefore remains a need for closed-cell water-blown rigid polyurethane foams which have reduced friability and acceptable adhesion to substrates but which also have acceptable compressive strength.